Recently, motor vehicles are usually equipped with speed alarm systems. In one of these speed alarm systems, an alarm signal is issued when the vehicle speed exceeds a predetermined value which usually corresponds to the maximum speed specified in the traffic rules. In another speed alarm system, the reference speed at which an alarm is issued may be variable and is arranged to be set by the vehicle driver at will. However, in these conventional speed alarm systems, the alarm signal is issued only when the vehicle speed exceeds (or equals) a reference value, and therefore, the driver of the motor vehicle can not ascertain whether the vehicle speed is adjacent to the reference speed unless the driver watches the speedometer of the vehicle.
Further, when the reference speed of the speed alarm system is variable, the speed alarm system may be used as an indicator which supplies the vehicle driver with a guide speed at which the vehicle driver intends to drive the vehicle, wherein the reference speed may be changed in accordance with the conditions of the traffic and the road. In this case the vehicle driver has to pay attention to the difference between the vehicle speed and the reference speed. Since the speed alarm system does not indicate this difference, the vehicle driver has to see the speedometer to ascertain the same. To see the difference, the vehicle driver has to see both of the reference speed and the actual speed of the vehicle which is indicated by the pointer of the speedometer. However, it is quite dangerous to see the speedometer continuously while driving especially at high speed.